Change acquired the subcommand from vagrant-help.

I installd the subcommand-plugin like vagrant-global-stuaus. 
I thought that I would like to complement the subcommand.
plz check this pullrequest.

$ vagrant plugin list
vagrant-global-status (0.1.4)
vagrant-lxc (0.7.0)

$ vagrant[TAB]...
box            global-status  help           package        provision      resume         ssh-config     suspend
destroy        halt           init           plugin         reload         ssh            status         up

thanks,
hiono